The invention concerns a communication system for transmitting signals having frequencies that lie within a transmission band and for receiving signals having frequencies that lie within a reception band. Such communication systems are used, for example, in mobile phones operating in full-duplex mode, where signals are transmitted and received simultaneously in two different frequency bands. These communication systems usually comprise a duplexer having a transmission branch and a reception branch, where both the transmission branch and the reception branch are connected to a common antenna.
The transmission branch and the reception branch need to be isolated from one another so that transmission signals and reception signals do not interfere with one another even though the transmission branch and the reception branch are connected to a common antenna. Signals with frequencies in the transmission band should be passed to the antenna but not to the reception branch. For these signals, the reception branch should be reflective, that is it should have a reflection coefficient close to unity. Similarly, signals having frequencies in the reception band should only be passed from the antenna to the reception branch but not to the transmission branch. The transmission branch should be reflective for signals with frequencies in the reception band.
The transmission branch and the reception branch also need to be impedance matched to the antenna in order to ensure a low insertion loss for the signals transmitted to the antenna and for the signals received by the antenna. However, the impedance of the antenna typically varies considerably over frequency. For UMTS, the transmission band lies between 1920 MHz to 1980 MHz and the reception band lies between 2110 MHz to 2170 MHz. Both the transmission band and the reception band have a bandwidth of 60 MHz and are separated from one another by 130 MHz. The impedance matching of the antenna is thus a compromise over all frequencies that lie in the 250 MHz band that covers both the transmission band and the reception band. As a result, the transmission branch and the reception branch are usually poorly matched to the antenna which reduces the efficiency of the communication system.